Glass-to-metal joint for a solar receiver

ABSTRACT

A glass-to-metal sealing device of a solar receiver has a metal collar and a glass cylinder to be sealed together. The glass cylinder is made of a borosilicate glass having a coefficient of thermal expansion within the range of [3.1, 3.5]·10 −6 ° C. −1  in a temperature range of [50, 450]° C. The metal collar s made of an austenitic alloy having a coefficient of thermal expansion in the range of [3.5, 6.0]·10 −6 ° C. −1  in the temperature range of [50, 450]° C. The end portion of the metal collar is beveled so as to increase its mechanical flexibility and, further, the end portion of the metal collar is processed via a thermal treatment in order to establish a bond between the metal and the glass surfaces.

The present invention relates to a glass-to-metal sealing device, to amethod of producing a glass-to-metal sealing device and to a tubularsolar receiver according to the preambles of claims 1, 11 and 12respectively.

A key component of parabolic trough CSP (Concentrated Solar Power) isthe Heat Collector Element (HCE) also known as solar receiver. One ofthe main issues that this element has to face and solve is the tightnessto preserve a suitable designed vacuum pressure in order to reducethermal losses to radiative phenomena only.

In a solar receiver, the most critical component undergoing possiblevacuum losses is the connection between glass and metal, also known asGlass-to-Metal Seal (GMS).

Parabolic trough CSP solar plants are designed to produce energy byconcentrating solar rays to a solar receiver, into which an HeatTransfer Fluid (HTF) flows through; the transfer fluid being heated upto high temperatures (up to 580-600° C.) and allowing, in a separatepower block, the production of steam and therefore of electricity via adedicated turbine.

For the thermodynamic cycle to properly work, the solar receiver has tomaximally absorb the concentrated solar rays and minimally release theheat. A spectral selective coating covering the stainless steel tube isoptimized in order to achieve high absorbance and low emissivity;furthermore, the thermal loss is minimized by encapsulating the tubeinto a vacuum environment by means of a co-axial cylindrical glass tube(having high optical transmittance).

Vacuum is mandatory to reduce thermal losses to radiative phenomenaonly.

A solar receiver will then necessarily contain two glass-to-metaltransitions, also known as Glass to Metal Seals (GMS), which indeedrepresent the most critical component to possible vacuum losses.

The GMS solutions developed for solar receiving tubes in the CSP worldhave been driven both by technology requirements as well as by marketand business needs.

FIG. 1 is schematically illustrating a solar receiver comprising aninner metal tube, not shown, and outer glass tube which are connectedtogether via two glass-to-metal seals 10 and via two metallic bellows,not shown, welded to the inner metallic tube. Each glass-to-metal seal10 comprises a metal collar 11, also known as metal cap or metal ring,and a glass cylinder 12 sealed together as schematically shown in FIG.3. The glass cylinder 12 is connected to the central glass portion 13 ofthe outer glass tube.

Several different types of glass-to-metal seals with various glasses andmetals with different thermal expansion coefficients and sealingtechniques are known in the art.

As used herein, the thermal expansion coefficient (TEC) of a material isdefined as the ratio between the elongation, ΔL, and the proper length,L, of a material when it undergoes a temperature change ΔT.

According to the disclosure of two US patents by Mr. Houskeeper in 1919(U.S. Pat. No. 1,293,411 and U.S. Pat. No. 1,295,466), it is known atechnique for compensating for the drawbacks caused by the difference inthe TEC coefficients of the glass and the metal, in which thehermetically sealing between glass and metal is improved by reducing thethickness of a portion of the metal element with a geometry as proposedby Mr. Houskeeper.

According to a known GMS technique, it has been developed a GMS jointbetween a stainless steel grade (aisi430) with a borosilicate glass ofthe family 3.3. Unfortunately, such GMS joint between Aisi430 steel and3.3 borosilicate glass suffers for the drawback of having a very largedifference between the values of the thermal expansion coefficients(TEC) of the metal and the glass, with a negative impact on the GMSunder mechanical forces induced by thermal variations. In fact, the TECvalues are: almost constant to 3.3·10⁻⁶° C.−1 for the glass and between[10,12]·10⁻⁶ ° C.−1 for the metal in the temperature range of [50,450]°C.

According to another known GMS technique (U.S. Pat. No. 7,562,655), ithas been developed a GMS joint between an austenitic alloy with welldefined concentrations of Nickel and Cobalt (commonly known asKovar-like alloy, DIN 17745, ASTM F15) with a borosilicate glass of thefamily 5.1. Unfortunately, such GMS joint has the drawbacks that Kovaris a pretty costly alloy (oscillating with Nickel market pricefluctuations) and that the 5.1 glass satisfying CSP dimensionalspecifications is still uncommon in the glass market.

According to other known techniques, transition glasses are adopted inGMS joints to limit the 5.1 glass to the sole GMS part (10), hencejoining together a kovar-to-5.1 solution to a 3.3 glass as shown in FIG.2. FIG. 2 is a drawing schematically illustrating a portion of the outerglass tube of the solar receiver comprising a matched GMS joint 10 witha different glass for the central glass portion 13 by employing a set oftransition glasses having different TECs, as for instance, a 1sttransition glass 21, a 2nd transition glass 22 and a n-th transitionglass 23, positioned between the GMS joint 10 and the central portion 13of the glass tube. Unfortunately, such GMS joints have the drawbacksthat transition glasses are expensive and the manufacturing process iscomplex.

Another known sealing technology, as for instance laser welding, iscomfortable but even more sensible to raw materials tolerances anddimensional specifications.

It is therefore the aim of the present invention to overcome the abovementioned drawbacks, in particular by providing a glass-to-metal sealingdevice, a method for producing a glass-to-metal sealing device and atubular solar receiver different from a fully matched solution (as forinstance the expensive and market uncommon kovar-to-5.1 glass) and froma transition glass solution (characterized by cheaper glass used onlyfor the central glass portion 13) via a direct GMS joint between a[3.1,3.5] TEC borosilicate glass and an austenitic alloy havingdifferent thermal expansion coefficients.

The aforementioned aim is achieved by a glass-to-metal sealing device ofa solar receiver, the device comprising a metal collar and a glasscylinder to be sealed together, the device further comprising thefollowing features:

-   -   a) the glass cylinder is made out of a borosilicate glass having        a thermal expansion coefficient in the range of [3.1,3.5]·10⁻⁶°        C.⁻¹ in the temperature range of [50,450]° C.;    -   b) the metal collar is made of an austenitic alloy having a        thermal expansion coefficient in the range of [3.5,6.0]·10⁻⁶°        C.⁻¹ in the temperature range of [50÷450]° C.;    -   c) the end portion of the metal collar is beveled so as to        increase its mechanical flexibility;    -   d) the end portion of the metal collar is processed via a        thermal treatment in order to establish a bond between the metal        and the glass surfaces.

The aforementioned aim is achieved also by a method of producing aglass-to-metal sealing device of a solar receiver, the device comprisinga metal collar and a glass cylinder (12) to be sealed together, themethod comprising the following steps:

-   -   a) providing, as glass for the glass cylinder, a borosilicate        glass having a thermal expansion coefficient in the range of        [3.1,3.5]·10⁻⁶° C.⁻¹ in the temperature range of [50,450]° C.;    -   b) providing, as metal of the metal collar, an austenitic alloy        having a thermal expansion coefficient in the range of        [3.5,6.0]·10⁻⁶° C.⁻¹ in the temperature range of [50÷450]° C.;    -   c) beveling the end portion of the metal collar so as to        increase its mechanical flexibility.    -   d) processing the end portion of the metal collar via a thermal        treatment for establishing a bond between the metal and the        glass surfaces;    -   e) sealing together the end collar portions of the glass        cylinder and the metal collar.

The aforementioned aim is achieved also by a tubular solar receiver inwhich the outer glass tube is connected to the inner metal tube via theglass-to-metal sealing device according to the proposed invention.

Embodiments of the invention enable to retain the designed vacuum forthe expected lifetime of the Heat Collector Element (HCE).Advantageously, the joint between the glass cylinder and the metal capshould be able to preserve a designed ultimate desired tightness, i.e. apressure of p<10⁻⁴ mbar, by fulfilling dedicated dimensionalrequirements so that the stability and durability of GMS joint isreliably ensured.

With embodiments of the invention, the glass component of the GMS jointundergoes mainly compressive stresses, reducing the dangerous tensilestresses to few MPa which is perfectly acceptable even by ordinary 3 mmthick glass tubes.

With embodiments of the invention, the dimensional output leads to a GMSproduct perfectly consistent with the typical working conditions of asolar plant, hence suitable for CSP applications.

Embodiments of the invention enable to achieve a simplification in themanufacturing process, both from the cost point of view as well as fromthe final performances achievable for the target solar receiver product.

Embodiments of the invention lead to industrial benefits in the field ofunmatched GMS products for the following reasons:

-   -   commodity of raw material: the 3.3 borosilicate glass is a well        know material and easy to find on the market;    -   performances of the glass: the 3.3 borosilicate glass easily        reaches a transmittance of 91.5-92.0%;    -   cost of the raw material: the 3.3 borosilicate glass is cheaper        than the 5.1 borosilicate glass;    -   simplification of manufacturing process: the manufacturing        process does not contain serious bottleneck steps so that        manufacturing costs are reduced.

Hence, embodiments of the invention lead to sensible cost reductions forsolar receivers, contributing to the decrease of the evaluated LevelizedCost Of Energy (LCOE) for solar energy in CSP parabolic trough plants.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a drawing schematically illustrating a solar receiver (Prior Art,previously described);

FIG. 2 a drawing schematically illustrating a portion of a solarreceiver comprising a GMS device and a set of transition glasses (PriorArt, previously described);

FIG. 3 a drawing schematically illustrating a GMS joint (Prior Art);

FIG. 4 a drawing schematically illustrating a metal collar according toan example embodiment of the present invention;

FIG. 5 a drawing schematically illustrating a GMS device according to anexample embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 4 is a drawing schematically illustrating a metal collar accordingto an example embodiment of the present invention.

FIG. 4( b) is a drawing schematically illustrating the end portion ofthe longitudinal section of the metal collar 11 circled in FIG. 4 a.

According to the proposed present invention, the proposed GMS joint 10between the metal collar 11 and the glass cylinder 12 is an unmatchedglass-to-metal sealing. In fact, the employed glass and metal materialsbehave differently under thermal gradients, especially when a physicalconnection between them, i.e. the sealing, has been established. Thecloser the corresponding TCE values, the softer the mechanical stress onthe overlap region. Additionally, the heating up or cooling downvelocity is surely different for glass and metal materials,independently whether matched or unmatched seals are considered.

The glass cylinder 12 is made out of a borosilicate glass having athermal expansion coefficient in the range of [3.1,3.5]·10⁻⁶ ° C.⁻¹ inthe temperature range of [50,450]° C. In a preferred embodiment, for theCSP field, a 3.3 borosilicate glass is used. Advantageously, such glasstype is easy to find in the market at commodity prices. The metal collar11 is made out of an austenitic alloy having a thermal expansioncoefficient in the range of [3.5,6.0]·10⁻⁶° C.⁻¹ in the temperaturerange of [50,450]° C. In a preferred embodiment of the invention, forthe CSP field, such austenitic alloy has suitable concentration ofNickel and Cobalt contents, according to the DIN 17745/ASTM F15 norms.In the field, the metals which fulfill such specifications are alsoknown as Kovar-like alloys.

The end portion of the metal collar 11 is beveled so as to increase itsmechanical flexibility. Advantageously, with such developed metal collargeometry, the tensions originated on the glass side of the GMS joint 10are decreased through a compensation of the glass stiffness with respectto the metal mobility.

Thus, such metal collar geometry characterized by elastic propertiesmitigates the glass stresses which can be produced on the (stiff) glasscomponent of the GMS joint by improving the mechanical elasticity of themetal.

Drawings of preferred embodiment examples are schematically illustratedin FIG. 4 and in FIG. 5.

The illustrated dimensions of FIG. 4 and FIG. 5 have been obtained bystudying the mechanical stresses involved in real working conditions ofa typical solar CSP plant. Preferred used materials are a 3.3borosilicate glass for the cylinder 12 and a Kovar-like alloy (DIN 17745, ASTM F15) for the metal collar 11.

FIG. 4( a) schematically illustrates a metal collar 11 according to anexample embodiment of the present invention.

FIG. 4( b) schematically illustrates a zoomed detail of the longitudinalsection of the free end portion FEP of the metal collar 11 as circled inFIG. 4( a). Such metal collar free end portion FEP will be then sealedto the glass cylinder as later described.

According to a preferred embodiment, as schematically shown in FIG. 4(b), the beveling of the end portion of the metal collar 11 is performedso as to obtain longitudinal sections having a trapezoidal-like shape inwhich the minor base m is at the free end side of the metal collar. Itis noted that, herein, with the term trapezoidal-like shape it is notintended only the trapezium shape itself but also similar shapes inwhich the sides are not totally straight or in which the two bases areparallel but more a bevel shape or a tooth-like shape in which the endportion has a reduced thickness with respect to the beginning portion.In fact, the term trapezoidal-shape has been herein used mainly forexplanatory purposes, i.e. for the sake of simplicity so as to describethe geometrical specifications in terms of bases and angles. In FIG. 4(c) are illustrated the major base M, the minor base m, the two lateralsides L1, L2 of the trapezoidal-like shape of the collar end. Thelateral side L1 is also representing the distance between the two basesm,M. The angle α, not shown, indicates the acute angle formed by the twolateral side L1, L2.

In invention embodiments, the following dimensions are recommended basedon studies on real stress conditions of typical CSP plants:

-   -   the ratio between the length of the minor base m and the length        of the major base M being greater than 0.25; and/or,    -   the length of the major base M in the range of [0.3, 0.6] mm and        the length of the minor base m being in the range of [0.15, 0.6]        mm; and/or,    -   the lateral sides L1,L2 forming an angle α in the range of        [0.5,10] degrees; and/or,    -   the maximum thickness T of the metal collar being in the range        of [0.3,1.2] mm.

For example, in a preferred embodiment, the length of the minor base mmay be 0.3, the length of the major base M may be 0.4, the length of thelateral side L1 may be 7 mm and the angle α may be 0.82 degrees.

According to the proposed invention, the end portion of the metal collar11 is processed via a thermal treatment for establishing a bond betweenthe metal and the glass surface of the GMS joint. Advantageously, adedicated structure on the metal surface suitable for a physical andchemical bond of the metal to the glass is achieved.

In fact, with such thermal treatment of the metal collar, a gridstructure on the metal is conveniently created so that the glassmaterial grips to the metal substrate (mechanical/physical join) and,simultaneously, a proper layer is suitably created on the metal surfacein order for the glass to bond to it (chemical bond).

According to a preferred embodiment of the invention, the thermaltreatment may preferably be an oxidation treatment to generate on themetal surface a glass-dedicated oxide layer.

Preferably, the glass-dedicated oxide layer thickness is tuned to be inthe range of [0.3,3.0] μm, with a penetration in the metal matrix in therange of [1.5,18.0] μm.

Additionally, according to another preferred embodiment, it isrecommended to develop an oxidation process characterized by a hydrogencontent limited to few percents in concentration (upto 5% in volume), inorder to minimize the sticking of hydrogen-atoms in interstitialposition within the crystalline structure (as hydrogen is one of themost difficult gas to be pumped away).

According to a preferred embodiment of the invention, theglass-dedicated oxide is an iron oxide. The iron oxide may preferably beeither FeO or Fe₃O₄ or a mixture of FeO and Fe₃O₄.

A controlled thermal cycle process is advised in order to achieve thedesired iron oxide as well as the optimum thickness and uniformity.

According to a preferred embodiment, the following thermal sealingprocess steps may be recommended for sealing the metal collar to theglass cylinder:

-   -   a heating step, in which controls on temperature and on rotation        are recommended;    -   a melting step, in which controls on temperature, on rotation        and on calibration of the molten glass edge are recommended;    -   a joining step for inserting the beveled metal into the molten        end portion of the glass cylinder, in which controls on        temperature, on rotation, on burner relative positions and on        mechanical produced forces (push/pull) on the will be final GMS        joint are recommended;    -   an in-line annealing step, in which careful control on        temperature decrease to achieve a glass temperature below its        characteristic softening temperature is recommended.

In order to avoid destructive effects due to small error propagations,the most critical thermal sealing sub-steps have been monitored andaccordingly some parameters have been identified as requiring particularattention in controlling their absolute values and behaviors as, forexample:

-   -   heating up rates should preferably be maintained within [6,35]°        C./sec,    -   cooling down rates should preferably be maintained within        [1.5,20]° C./sec,    -   rotational speed should preferably be maintained within [12,100]        rpm,    -   burner should preferably be adjustable in a distance range of        [−5.5,5.5]mm from the glass edge, depending on the considered        process step, and at a translational speed within [0,15] mm/sec.

FIG. 5( b) is a drawing schematically illustrating, according to anexample embodiment, the circled detail of the GMS joint of FIG. 5( a).

According to a preferred embodiment, the end portion of the glasscylinder 12 is melted via a dedicated thermal process so as to form, atthe glass edge, an enlarged molten glass having a sphere-like shape,hereinafter denoted as molten glass ball GB. As shown in FIG. 5( b) theglass edge has a maximum thickness of circa 12.6 mm (Gi+Ge+m) while thethickness G_(T) of the glass cylinder away from the glass ball GB is aregular glass thickness of circa 3 mm.

In preferred embodiments, the following dimensions for the GMS jointsare advantageously recommended, where we denote by “internal” side andby “external” side the side looking towards the symmetry axis of thesolar tubular receiver and the side looking towards the outeratmosphere, respectively:

-   -   Axial (linear) extensions of the glass-to-metal overlap internal        and external O_(i), O_(e): in the range of [2,5.0] mm, and/or    -   Glass thicknesses internal and external G_(i), G_(e) in the        range of [3.0,6.0] mm; and/or,    -   Moreover, it is also recommended to end up with a smooth,        smaller than 90° contact angle β_(i),β_(e)defined as the angles        measured at the contact interfaces between glass and metal in        FIG. 5( b).

Since the previously described thermal sealing process involves veryhigh temperatures on the metal collar and on the glass cylinder(exceeding the softening and the melting points), unavoidable tensionsmight get stacked at the interface between the two materials.

As these tensions could bring to GMS micro breakage, i.e. leaking of theGMS, during the further thermal cycles typical of successive solarreceiver production steps as well as in real life working conditions, itis recommended, in a preferred embodiment, to implement an off-lineannealing process whose goal is to get rid of possible glass residualstresses in the overlap region.

Hence, the above mentioned tensions can be advantageously smeared intheir intensity over a wider region, decreasing therefore theirpotential dangerous impact. The cooling down rate value may convenientlybe set in the range of [0.4,2.5]° C./min.

In embodiments of the invention, as part of a vacuum device, the glasscylinders 12 and the metal caps 11 may be properly cleaned, bydeveloping a dedicated recipe in order to avoid undesired contaminants,however without applying strong chemical polishing which can causeunwanted nano-scratches on the glass surfaces. Under cleaning procedureshould also be intended the ways the GMS joints are stored, aiming atavoiding contamination with humidity and greasy atmosphere during thestoring procedure.

In addition to the embodiments of the present invention described above,the skilled persons in the art will be able to arrive at a variety ofother arrangements and steps which, if not explicitly described in thisdocument, nevertheless fall within the scope of the appended claims.

LIST OF USED ACRONYMS

CSP Concentrated Solar Power

GMS Glass-to-Metal Seal

HCE Heat Collector Element

HTF Heat Transfer Fluid

LCOE Levelized Cost Of Energy

TEC Thermal Expansion Coefficient

1-12. (canceled)
 13. A glass-to-metal sealing device of a solarreceiver, the device comprising: a metal collar and a glass cylinder tobe sealed to one another; said glass cylinder being formed of aborosilicate glass having a coefficient of thermal expansion in a rangeof [3.1, 3.5] 10⁻⁶° C.⁻¹ in a temperature range of [50, 450]° C.; saidmetal collar being formed of an austenitic alloy having a coefficient ofthermal expansion in a range of [3.5, 6.0] 10⁻⁶° C.⁻¹ in the temperaturerange of [50, 450]° C.; said metal collar having a beveled end portionfor increasing a mechanical flexibility thereof; and said end portion ofsaid metal collar being processed via a thermal treatment in order toestablish a bond between the metal of said metal collar and glasssurfaces of said class cylinder.
 14. The glass-to-metal sealing deviceaccording to claim 13, wherein said beveled end portion of said metalcollar is defined by longitudinal sections having a trapezoidal shapewith a minor base at a free end of said metal collar.
 15. Theglass-to-metal sealing device according to claim 14, wherein a ratiobetween a length of the minor base and a length of a major base of thetrapezoidal shape is greater than 0.25.
 16. The glass-to-metal sealingdevice according to claim 15, wherein the length of the major base liesin a range of [0.3, 0.6] mm and the length of the minor base lies in arange of [0.15, 0.3] mm.
 17. The glass-to-metal sealing device accordingto claim 14, wherein lateral sides (L1, L2) of the trapezoidal shapeenclose an angle in a range of [0.5, 10] degrees.
 18. The glass-to-metalsealing device according to claim 13, wherein said austenitic alloy ofsaid metal collar has a concentration of nickel and cobalt contents tosatisfy an ASTM F15/DIN 17745 norm.
 19. The glass-to-metal sealingdevice according to claim 13, wherein the thermal treatment processingsaid end portion of said metal collar is an oxidation treatmentgenerating on the metal surface a glass-dedicated oxide layer.
 20. Theglass-to-metal sealing device according to claim 19, wherein theglass-dedicated oxide layer has a thickness spanning a range of [0.3,3.0] μm and a penetration in a metal matrix in a range of [1.5,18.0] μm.21. The glass-to-metal sealing device according to claim 20, wherein theglass-dedicated oxide is an iron oxide.
 22. The glass-to-metal sealingdevice according to claim 21, wherein the iron oxide is selected fromthe group consisting of FeO, Fe₃C₄, and a mixture of FeO and Fe₃C₄. 23.A method of producing a glass-to-metal sealing device of a solarreceiver, the device including a metal collar and a glass cylinder to besealed together, the method comprising the following steps: a)providing, as a glass for the glass cylinder, a borosilicate glasshaving a coefficient of thermal expansion in a range of [3.1, 3.5] 10⁻⁶°C.⁻¹ in a temperature range of [50, 450]° C.; b) providing, as a metalof the metal collar, an austenitic alloy having a coefficient of thermalexpansion in a range of [3.5, 6.0] 10⁻⁶° C.⁻¹ in the temperature rangeof [50, 450]° C.; c) beveling an end portion of the metal collar so asto increase a mechanical flexibility thereof; d) processing the endportion of the metal collar via a thermal treatment for establishing abond between the metal and glass surfaces; and e) sealing together theend collar portions of the glass cylinder and the metal collar.
 24. Atubular solar receiver, comprising an outer glass tube and an innermetal tube connected to one another via the glass-to-metal sealingdevice according to claim 13.